Negative optimum delay in Open Rocket, bug in Estes Prowler ork file?

I downloaded the attached Estes Prowler file from Rocket Reviews (just changed from rkt to ork), don't see any obvious flaws in the configuration but something must be wrong, because the sim results are really strange. Note the negative minimum optimum delays, the 0.021 stability, and the difference between the apogee in the blue-highlighted row (176') and the lower left (2795'). Also odd that the readings are normal with the F27 motor but obviously incorrect in the others.

Any thoughts?



View attachment Estes Pro Series II Prowler.ork

It’s unstable with the G motors; look at CP and CG. Those crazy sims are what you get when it skywrites. That said I don’t know why it says 2795’ in the lower left.

The actual rocket is stable, over 1.3 even with G motors. This file must have the weights messed up somehow, although the total weight is about right.

Easy correction then is just to weigh the rocket, find the CG, and override them in OR. Then you should be good to go, and don’t need to try to go through every part in the sim looking for the error(s).

It is rare to find an accurate sim file on Rocket Reviews. People try hard, but ....

Anyway, even an accurate sim is accurate only for the actual rocket that was modeled. You will have used more or less glue, etc. As Neil W. stated, whether you create the sim from scratch or borrow one, just do the overrides. But you may also want to measure the parts and compare to the sim, because if they are "close but no cigar," then the CP is wrong, too.

Without downloading the file and just eyeballing the screenshot, does the CP look a little bit forward of where you would expect it? Maybe not, but I would have guessed it would be further aft.

I agree with the advice to check the dimensions of the external parts, so you get an accurate CP. Then weigh the rocket without motors, and balance it without motors to find the CG. And then do a global override for mass and CG for the entire stage. You don’t need to worry about mass and CG of individual parts for accurate flight sims, just mass and CG of the entire rocket, and accurate external dimensions for an accurate CP.

And if it still looks like the G motors are too heavy for stability, you could add some nose weight, but if you add too much, it won’t fly well on F15 motors. So you might want an extra nose cone, so you can swap the weighed one in and out, depending on the motor.

I've usually started my sims by downloading an rkt or ork from a public source and then modifying it to match the specs on my particular rocket, but after this I'm thinking it may be better to always start a new file from scratch. What's particularly weird about this one is that the F27 sims normally but all others, including the F15 (an Estes "recommended motor" for this rocket), are way off.

As long as the External Dimensions of your downloaded file is fine, just do an override.

Check the simulation/plot settings too. Don't know is something was set differently than you're used to.

billdz said:

I've usually started my sims by downloading an rkt or ork from a public source and then modifying it to match the specs on my particular rocket, but after this I'm thinking it may be better to always start a new file from scratch. What's particularly weird about this one is that the F27 sims normally but all others, including the F15 (an Estes "recommended motor" for this rocket), are way off.

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There is nothing "weird" nor "way off" in the simulations. They are exactly correct with the data given. Did you read post #2 and the warnings issued by OR? The last 3 motors are heavier, move the CG rearward, and have unstable flights. You have wind in the simulations, adding to the instability off the rod. Not that hard to figure out. You did pass the L2 exam and provide simulations for L3, right?

If the "actual" margin is 1.3 (not sure how you know the actual CP of a built rocket, but whatever), then make it so with the overrides as many people already mentioned.

The file has mass overrides for practically everything except the nose cone and body tube, and since it says the stability is close to zero, the sim results are going to be nonsense. The F27 works because it's the most stable configuration, the others just aren't stable.

Does the nose cone really weigh 0.119#? Does the BT really weigh 0.151#? Is the fin weight right? If I add an ounce of weight to the nose then the results are much better. I suspect the NC is really heavier than the file says.

I measured and weighed my parts and this sim is actually really close. As Mike mentioned, the addition of weight in the NC solves the problem. I must be missing something, though, because this is a quite popular rocket and none of the reviews mentions unstability or the need to add NC weight. Next I'll try making a new sim from scratch. I'm sure Estes did not make a rocket with 0.2 stability on the recommended motor.

billdz said:

I'm sure Estes did not make a rocket with 0.2 stability on the recommended motor.

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Openrocket uses the conservative pure Barrowman formulas to compute CP; I would expect the CP to move back some, maybe a full caliber, using Rocksim, which uses a modified Barrowman formula. That may be part of it.

Once you've built the rocket, see if the CG is really where OR says it is. The OR file has the parachute unrealistically close to the motor, but that won't make much difference since it doesn't weigh much.

Estes has marketed marginally stable rockets before, and the F15 is pretty slow off the rod, but I wouldn't expect this design to be as bad as the OR model predicts.

billdz said:

I measured and weighed my parts and this sim is actually really close. As Mike mentioned, the addition of weight in the NC solves the problem. I must be missing something, though, because this is a quite popular rocket and none of the reviews mentions unstability or the need to add NC weight. Next I'll try making a new sim from scratch. I'm sure Estes did not make a rocket with 0.2 stability on the recommended motor.

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Weighing parts is a design step, what you do to design a stable rocket. Flight simulations need the numbers as built and as flown. Forget about weighing parts at this point. Weigh the unloaded rocket and measure its point of balance. That and correct dimensions for the outside components shoul give you accurate flights, but you also need to check things like wind and length of rail, if those were saved in the file you downloaded.

what IS strange; I downloaded both the .rkt file AND the .ork file the ork version behaves as the op described, the rkt file does not. what I find even stranger is; if both files are accurately depict the rocket...how did it make it out the door with such a poor cg/cp ratio, thought that the Estes policy was 1.5 cal stability ready to fly? even with the F motors it is only .2 cal. I suspect that someone goofed either Estes or the design file(the design file gets my vote as the culprit). could the op measure the cg sans engine and report back please. thanks
Rex